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How SCSI Works


Controllers, Devices and Cables
Internal SCSI devices connect to a ribbon cable.
Internal SCSI devices connect to a ribbon cable.
External SCSI devices connect using thick, round cables.
External SCSI devices connect using thick, round cables.

A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus.

Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.

68-pin Alternative 3 SCSI connector
68-pin Alternative 3 SCSI connector

Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. (Serial Attached SCSI devices use SATA cables.) In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device.

The cable itself typically consists of three layers:

  • Inner layer: The most protected layer, this contains the actual data being sent.
  • Media layer: Contains the wires that send control commands to the device.
  • Outer layer: Includes wires that carry parity information, which ensures that the data is correct.

Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors.

Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be closed. We'll look at how to do this next.